Afaik, insulating above a shallow field would be hugely detrimental to the field as the sun is greatly responsible for reheating the ground.
Additionally, if you are considering doing a field in series, be sure to have the panels before it returns to the field. Most heat pumps have a limit on the temperature of the incoming ground loop of around 110. I am not sure why this is, but I've seen it on the specs of every heat pump I've looked at.
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